Popping from light switches and refrigerator

I had a similar--(I think)-- situation to that you describe. It turned out to be the common behind the meter where the power comes in from the street.

All your circuts share a COMMON Ground. A computer grade or medical grade circut could be pulled but the simple solution is to isolate a single circut from the 4 that you have and ground that one on a dedicated or non-shared ground. It should cost very little. This will stop the bleed. You will want to make sure that nothing else with motors or pumps are on that line. Most electricians are not aquanted with Audio Grade lines. You will find that you must be specific and might have to use more than one electrician before finding one that understands what you really want. The problem can also happen when the 3 prong outlets are placed in the socket and a ground is established with a metal strip that universally grounds each outlet to a common ground. Make sure that if they wire it is a NEW 3 WIRE Feed wire without a common ground.

The popping noise can just be line noise as the engineer mentioned to you. When you break current flow suddenly this creates an inductive kick (short duration voltage spike) that needs to bleed off somewhere. I've got 10 gauge solid copper dedicated lines from my breaker box to my stereo, a copper water pipe based grounding system that should satisify even the most finatic, and I still hear the pop from one heavily loaded light switch circuit. An alternative might be to have an electrician install surge supressors at your breaker box, this should be pretty cheap.
For what it is worth, we had an intermittent short on a piece of equipment at work that drove us insane for a while. It was on a circuit fed from a large (80 breakers) panel, the short wasn't enough to open a breaker but it would give enough of a spike to blow the most voltage sensitive equipment fed from the panel every now and then.
Anyway, were its my stuff, I'd -
1. Take the measuresments mentioned in the note above & if possible hang a brush recorder or oscilloscope on the line & see if there is anything that comes and goes.
2. Wonder if a short inside the amp is what made your audio life sour, and be anxious to find out what is wrong with it from the shop.
3. Consider a dedicated line for the long term as mentioned above.
4. Once you get data from 1-3 the answer might be obvious.
On the bright side, problems like this are always very simple after you know the answer.